It will be appreciated that faults in RF transmission lines and waveguides require detection so that the distance to the fault may be calculated. Moreover, the ability to detect and locate multiple faults that oftentimes result in ghosts that resemble faults requires sophisticated techniques to eliminate the need for skilled technicians.
Nowhere are reflectometers more desirable than in aircraft applications in which long lengths of cable or waveguides are used, both for the control of the aircraft and for the electronics devices such as radar or communication systems carried by the aircraft. Additionally, civilian use of reflectometers finds application in cable systems in which faults are detected based on the distance from a cable plant.
Likewise, for satellite and terrestrial-based communications, RF transmissions lines and waveguides can become faulty, especially at the interface between two cables as at a connection, and the ability to identify and locate the fault is a paramount concern.
A fault is anything that causes a change of impedance in the material properties of the cable or waveguide that causes some of the energy that is being transmitted through the transmission line to be reflected. Typically the faults occur at the interface between two cables, for instance, if a connection is not torqued properly or the connectors are old. If there is no good interface there will be a difference in material properties at the interface so that when one is sending a signal down the transmission line, at the interface some of the energy will be reflected. Thus, cable discontinuities or faults can be the result of interface problems between the cables or when a cable is bent and tweaked enough, the material properties will change at the bend that cause a reflection. Also, if the shielding to the cable is damaged in any way, part of the cable may couple to, for instance, the body of an aircraft, causing an impedance mismatch.
Thus, in the past, whether it be for military applications so that one does not have to tear into a large amount of cabling in an aircraft in order to locate a fault, or whether one is testing cell tower base stations to locate faults in transmission lines for the cell towers, or if one is using a distance-to-fault (DTF) detector for satellite ground terminals, there is a need to be able to automatically and accurately identify the fault, to find its severity and to obtain the distance to the fault, especially in a multi-fault environment.
There are two primary methods of determining faults in any transmission line. The first is based on time domain reflectometry and the other is based on frequency domain reflectometry. In time domain reflectometry one can simply place a pulse on a transmission line and measure the time it takes for a return pulse to be received. Propagation time within the cables therefore determines the distance of the fault to a measuring plane. Thus, by knowing the velocity of propagation of the specific cable, one can convert the time that the reflected pulse comes back to a distance to know how far away the fault is.
It is noted that time domain reflectometry is particularly well adapted for low frequency applications because one can easily create a system that produces short duration pulses that are narrow enough to resolve fault locations with acceptable accuracy. However, when one starts to get into high frequency applications exceeding for example 1 GHz, obtaining a physical system to produce an impulse that is of short enough duration is very difficult to realize in hardware. Moreover, even if one can create such a short pulse, the shape of the pulse is hard to control.
It is noted that those employing time domain reflectometry do not obtain information on spectral characteristics. With frequency domain reflectometry one has full control of the spectral characteristics. Thus, the other technique for reflectometry, which has proved to be quite useful, involves frequency domain reflectometry in which a couple of very different approaches have been used.
As illustrated in U.S. Pat. No. 4,630,228, what is described is a frequency difference method in which a swept source is applied to a device under test (DUT) and the reflected wave is mixed down and measured. Then the distance to the fault is determined by the difference frequency.
Another approach is illustrated in U.S. Pat. No. 5,068,614, which is the automation of a manual distance to fault or DTF technique that uses a spectrum analyzer and an offset tracking generator to find the distance to a fault. The output of the tracking generator is mixed with the return from the device under test. The tracking generator offset frequency is then adjusted to maximize the power displayed on the spectrum analyzer, with the distance to the fault calculated from the offset frequency. Note that this system also employs a frequency difference method.
As illustrated in U.S. Pat. Nos. 5,949,236 and 5,994,905, the systems employed are not frequency difference systems. Rather they measure the reflection coefficient of the device under test as a function of frequency. The reflection coefficient varies as a function of frequency. This variation is based on the location of each fault and the percent of energy reflected from each fault. If a device under test contains a single fault the reflection coefficient will vary as an exponentially decaying sinusoid across frequency. The method employed is to use an Inverse Fourier Transform or IFT to obtain an impulse or time domain response that is then used to determine the distance to the fault.
The problem with the methods described in these two patents is that they only measure the amplitude of the reflection coefficient, not the phase. This introduces non-linearities that generate harmonics and intermodulation products in the impulse response. These undesirable responses do not occur if there is only one fault. However, the number of spurious responses increases exponentially with multiple faults. These spurious responses are sometimes called ghosts.
In short, there are an extremely large number of spurious responses when a device under test contains several faults and removing them all becomes both cumbersome and, in some cases, impossible, at least by visual inspection of the results of an Inverse Fourier Transform (IFT).
To make matters somewhat worse, when one seeks to measure distance to fault over a wide operating frequency range, typically in the past the measured signals are down-converted to a single frequency range from which Inverse Fourier Transform measurements are obtained. Down-conversion adds its own set of problems and involves many local oscillators and mixers.
Additionally, when multiple faults are involved, the energy transmitted through the first fault will be of considerably lower amplitude when it reaches a subsequent fault. Thus, the apparent reflection from a subsequent fault, which may be more highly reflective than the first, oftentimes has a magnitude much less than the amplitude of the peak associated with the first fault due to the attenuation of the signal that gets by the first fault, and also due to line attenuation. The result is that one may fail to recognize a subsequent stronger fault. Thus, those systems that do not adjust for prior faults are incapable of distinguishing, in subsequent faults, the severity of the fault.
The result is that it takes an extremely skilled technician to be able to recognize that a certain peak is the result of a fault, especially when the peaks start to fall into the noise level due to the amplitude attenuation associated with the line itself. Secondly, those systems that use an inverse amplitude attenuation function to compensate for attenuation tend to mask the faults at greater and greater distances.
There are some frequency domain reflectometers that rely on phase shifts to determine the distance to a fault. However, those systems that measure phase shift only which are based on real phase shifts, have no attenuation information. While they do take into account phase shifts per unit length of transmission line, they fail to take into account attenuation per unit length of transmission line.
Finally, those systems that use an Inverse Fourier Transform without further processing are incapable of subtracting out the effect of previous faults when trying to identify or locate peaks useful in determining distance to a fault; or in determining the severity of a fault.
By way of further background, in order for some of the prior systems to be able to obtain a wide operational frequency bandwidth, some prior systems use a stepped frequency approach and down-conversion, in which one transmits a sine wave at a set number of frequency points within the defined bandwidth. At each frequency point one is radiating a sine wave and one seeks to be able to measure the amplitude and phase of the returned sine wave. In so doing, one obtains a discrete frequency response of the cable or waveguide. If one then performs an Inverse Fourier transform one obtains a temporal response, at which point one can obtain the same results as time domain reflectometers.
The trouble is that for a wide bandwidth, the prior art systems use down-convert, down-mix or heterodyning circuits, employing a local oscillator to down-mix to a signal that one can sample. One then needs to sample at one frequency range and then step through the remainder of the frequency ranges to obtain the wide bandwidth. For a wide bandwidth system it would be desirable to eliminate oscillators and down-converters and to eliminate a great many stepping functions.
More particularly, when one utilizes down-conversion, one wants to measure a vector voltage. In other words, one wants to measure the phase and amplitude. This is a relatively easy task at a single frequency, but the problem is, as soon as one starts trying to do it over a wide range of frequencies, devices become non-ideal and they are hard to calibrate. Thus, measuring a device over a wide frequency range is very difficult. In order to be able to do so, one down-converts to a single frequency and makes the measurements at this frequency.